Unico Campania and Campania d'Arte passes

We are spending a week in Sorrento, after flying into Rome on May 10th. I'd like to use the Unico Campania pass to get from Naples to Sorrento. After we're settled in Sorrento, we want to use it for getting to all the Campania sites, along with the free and discounted entrances offered by the Campania d'Arte. Is this a good idea? How do I buy these passes before May 9th?